Method and apparatus for cancelling self-interference signal between transmission antenna and reception antenna

ABSTRACT

An apparatus for cancelling a self-interference signal between a transmission antenna and a reception antenna is disclosed. The apparatus includes a first self-interference signal cancellation unit for cancelling a self-interference signal in consideration of a linear channel between the transmission antenna and the reception antenna, a second self-interference signal cancellation unit for cancelling a self-interference signal in consideration of nonlinear channel characteristic between the transmission antenna and the reception antenna or linear characteristic of a radio channel, and a controller for comparing a transmitted signal output from the transmission antenna and a received signal received by the reception antenna to provide a first coefficient to be applied to self-interference signal cancellation of a linear device in the first self-interference signal cancellation unit and a second coefficient to be applied to self-interference signal cancellation of a nonlinear device in the second self-interference signal cancellation unit.

Pursuant to 35 U.S.C. §119(e), this application claims the benefit ofU.S. Provisional Application No. 61/984,842, filed on Apr. 27, 2014,which is hereby incorporated by reference as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a method and apparatus for cancelling aself-interference signal between a transmission (Tx) antenna and areception (Rx) antenna.

2. Discussion of the Related Art

A full duplex radio (FDR) or full duplex communication scheme refers toa communication scheme for simultaneously supporting transmission andreception using the same resource in one user equipment (UE). In thiscase, the same resource refers to the same time and the same frequency.FDR communication or full duplex communication is referred to as two-waycommunication.

FIG. 1 is a diagram illustrating concept of a UE and a base station(BS), which support FDR. Referring to FIG. 1, in a network state thatsupports FDR, there are three types of interferences. First interferenceis intra-device self-interference. The intra-device self-interferencerefers to interference caused by signals that are transmitted from atransmission (Tx) antenna and received by a receiving (Rx) antenna inone BS or UE. Since the signals transmitted from the Tx antenna aretransmitted with high power and a distance between the Tx antenna andthe Rx antenna is small, the transmitted signals are received by the Rxantenna while attenuation is barely caused, and thus, are received withhigher power than a desired signal. Second interference is UE to UEinter-link interference. In a network that supports FDR, the UE to UEinter-link interference is increasingly caused. The UE to UE inter-linkinterference refers to interference caused by uplink signals that aretransmitted from a UE and received by an adjacently positioned UE. Thirdinterference is BS to BS inter-link interference. Similarly, in anetwork state that supports FDR, BS to BS inter-link interference isincreasingly caused. The BS to BS inter-link interference refers tointerference caused by signals that are transmitted between BSs orheterogeneous BSs (pico, femto, and relay) in a HetNet state andreceived by an Rx antenna of another BS.

Among the three types of interferences, the intra-deviceself-interference (hereinafter, referred to as self-interference) isinfluence of interference caused only in FDR. In order to manage FDR, amost serious problem is cancellation of self-interference. However,methods for effectively cancelling self-interference in an FDR statehave not been discussed in detail.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus forcancelling a self-interference signal between a transmission (Tx)antenna and a reception (Rx) antenna.

Another object of the present invention is to provide a method forcancelling a self-interference signal between a Tx antenna and a Rxantenna.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, anapparatus for cancelling a self-interference signal between atransmission antenna and a reception antenna includes a firstself-interference signal cancellation unit for cancelling aself-interference signal in consideration of a linear channel betweenthe transmission antenna and the reception antenna, a secondself-interference signal cancellation unit for cancelling aself-interference signal in consideration of nonlinear channelcharacteristic or linear characteristic of a radio channel between thetransmission antenna and the reception antenna, and a controller forcomparing a transmitted signal output from the transmission antenna anda received signal received by the reception antenna to provide a firstcoefficient to be applied to self-interference signal cancellation of alinear element in the first self-interference signal cancellation unitand a second coefficient to be applied to self-interference signalcancellation of a nonlinear element in the second self-interferencesignal cancellation unit. The second self-interference signalcancellation unit may cancel a self-interference signal of a nonlinearelement using the second coefficient. The first self-interference signalcancellation unit may cancel a linear self-interference signal inconsideration of time taken to receive the transmitted signal by thereception antenna using the first coefficient. The firstself-interference signal cancellation unit and the secondself-interference signal cancellation unit may be configured in seriesbetween a receiver including the transmission antenna and a receiverincluding the reception antenna. The controller may be configuredbetween the transmission antenna and the reception antenna. Thecontroller, the first self-interference signal cancellation unit, andthe second self-interference signal cancellation unit may be connectedand configured in series between a node between a band pass filter and atransmission antenna at a transmitter and a node between a band passfilter and a reception antenna at a receiver.

In another aspect of the present invention, a method for cancelling aself-interference signal between a transmission antenna and a receptionantenna includes comparing a transmitted signal output from thetransmission antenna and a received signal received by the receptionantenna to provide a first coefficient to be applied toself-interference signal cancellation of a linear element and a secondcoefficient to be applied to self-interference signal cancellation of anonlinear element, cancelling a linear self-interference signal usingthe first coefficient, and cancelling a nonlinear self-interferencesignal using the second coefficient. The cancelling of the linearself-interference signal may include cancelling a self-interferencesignal in consideration of time taken to receive the transmitted signalby the reception antenna using the first coefficient.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a diagram illustrating concept of a user equipment (UE) and abase station (BS), which support full duplex radio (FDR);

FIG. 2 is a block diagram illustrating a structure of BS and a UE 110 ina wireless communication system;

FIG. 3 is a diagram illustrating concept of self interference;

FIG. 4 is a diagram illustrating signal distortion due to quantizationerrors and FIG. 5 is a diagram illustration signal recovery whenquantization errors are low;

FIG. 5 shows an example in which an interference signal has lower powerthan a desired signal and the desired signal is recovered after theinterference signal is cancelled;

FIG. 6 is a diagram for explanation of a scheme for cancellingself-interference;

FIG. 7 is a diagram for explanation of an antenna interferencecancellation (IC) scheme using a distance between antennas;

FIG. 8 is a diagram for explanation of an antenna IC scheme using aphase shifter;

FIG. 9 illustrates interference cancelling performance according to abandwidth and center frequency of a signal;

FIG. 10 is a diagram illustrating a system obtained by combininginterference cancellation (IC) schemes;

FIG. 11 is a block diagram for interference cancellation in anenvironment in which orthogonal frequency division multiplexing (OFDM)is used;

FIG. 12 is a diagram illustrating S parameter characteristic of anantenna for a frequency;

FIG. 13 is a diagram illustrating frequency versus antenna gaincharacteristic;

FIG. 14 is a diagram illustrating an example of an analog cancellationscheme;

FIG. 15 is a block diagram for reflection of nonlinear distortioncharacteristic;

FIG. 16 is a diagram illustrating an analog cancellation scheme thatconsiders nonlinear characteristic of a self-interference signalaccording to the present invention;

FIG. 17 is a block diagram illustrating an example of an analogcancellation scheme and illustrates the case in which the analogcancellation scheme is extended, Q=2, and K=2;

FIG. 18 is a diagram illustrating an analog cancellation scheme thatconsiders nonlinear characteristic of a self-interference signalaccording to the present invention; and

FIG. 19 is a diagram illustrating an analog cancellation scheme thatconsiders nonlinear characteristic of a self-interference signalaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the exemplary embodiments of thepresent invention with reference to the accompanying drawings. Thedetailed description, which will be given below with reference to theaccompanying drawings, is intended to explain exemplary embodiments ofthe present invention, rather than to show the only embodiments that maybe implemented according to the invention. The following detaileddescription includes specific details in order to provide a thoroughunderstanding of the present invention. However, it will be apparent tothose skilled in the art that the present invention may be practicedwithout such specific details. For example, the following descriptionfocuses upon a case in which a mobile communication system is a 3rdgeneration partnership project (3GPP) long term evolution (LTE) systemor a LTE-advanced (LTE-A) system. However, the present technicalfeatures, aside from unique features of 3GPP LTE and LTE-A may beapplied to any other mobile system.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention andimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

In addition, in the following description, it is assumed that a userequipment (UE) refers to any mobile or fixed type device of a user side,such as a user equipment, a mobile station (MS), an advanced mobilestation (AMS), etc., and that a base station (BS) refers to any node ofa network side that communicates with the UE, such as a Node B, an eNodeB, a base station, access point (AP), etc. Throughout thisspecification, the technical features of the present invention aredescribed based on an institute of electrical and electronic engineers(IEEE) 802.16 system, but may be applied to various other communicationsystems.

In a mobile communication system, a UE may receive information from a BSin downlink and transmit information in uplink. The informationtransmitted or received by the UE may be data and various controlinformation. In addition, there are various physical channels accordingto the type or use of the information transmitted or received by the UE.

The following technical features can be applied to a variety of wirelessaccess technologies, for example, code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), and the like. CDMAmay be embodied through radio technology such as universal terrestrialradio access (UTRA) or CDMA2000. TDMA may be embodied through radiotechnology such as global system for mobile communication (GSM)/generalpacket radio service (GPRS)/enhanced data rates for GSM evolution(EDGE), etc. OFDMA may be embodied through radio technology such as IEEE802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, evolved UTRA (E-UTRA),and the like. UTRA is a part of a universal mobile telecommunicationssystem (UMTS). 3GPP LTE is a part of evolved UMTS (E-UMTS), which usesE-UTRA. The 3GPP LTE employs OFDMA in downlink and employs SC-FDMA inuplink. LTE advanced (LTE-A) is an evolved version of 3GPP LTE.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to other formats within the technical scope orspirit of the present invention.

FIG. 2 is a block diagram illustrating a structure of a base station(BS) 105 and a user equipment (UE) 110 in a wireless communicationsystem 100.

Although one BS 105 and one UE 110 (including a D2D UE) are illustratedin order to simply illustrating the wireless communication system 100,the wireless communication system 100 may include one or more BSs and/orone or more UEs.

Referring to FIG. 2, the BS 105 may include a transmission (Tx) dataprocessor 115, a symbol modulator 120, a transmitter 125, a Tx/Rxantenna 130, a processor 180, a memory 185, a receiver 190, a symboldemodulator 195, and a reception (Rx) data processor 197. In addition,the UE 110 may include a Tx data processor 165, a symbol demodulator170, a transmitter 175, a Tx/Rx antenna 135, a processor 155, a memory160, a receiver 140, a symbol demodulator 145, and an Rx data processor150. Although FIG. 2 illustrates that each of the BS 105 and the UE 110includes the Tx/Rx antennas 130 and 135, respectively, each the BS 105and the UE 110 includes a plurality of Tx/Rx antennas. Accordingly, theBS 105 and the UE 110 according to the present invention support amultiple input multiple output (MIMO) system. In addition, the BS 105according to the present invention may support both single user-MIMO(SU-MIMO) and multi user-MIMO (MU-MIMO) schemes.

In downlink, the Tx data processor 115 receives traffic data, formatsand codes the received traffic data, and interleaves and modulates (orsymbol-maps) the coded traffic data to provide modulated symbols (“datasymbols”). The symbol modulator 120 receives and processes the datasymbols and pilot symbols to provide a stream of symbols.

The symbol modulator 120 multiplexes the data and pilot symbols andtransmits the multiplexed data and pilot symbols to the transmitter 125.In this case, each transmitted symbol may be a data symbol, a pilotsymbol, or a zero signal value. In each symbol period, pilot symbols maybe consecutively transmitted. The pilot symbols may each be a frequencydivision multiplexing (FDM) symbol, an orthogonal frequency divisionmultiplexing (OFDM) symbol, a time division multiplexing (TDM) symbol,or a code division multiplexing (CDM) symbol.

The transmitter 125 receives the stream of symbols, converts the streaminto one or more analog signals, and further adjusts (e.g., amplifies,filters, and frequency-upconverts) the analog signals to generate adownlink signal appropriate for transmission via a radio channel. Thenthe Tx antenna 130 transmits the generated downlink signal to the UE110.

In the structure of the UE 110, the Rx antenna 135 receives the downlinksignal from the BS 105 and provides the received signal to the receiver140. The receiver 140 adjusts (e.g., filters, amplifies, andfrequency-downconverts) the received signal and digitizes the adjustedsignal to acquire samples. The symbol demodulator 145 demodulates thereceived pilot symbols and provides the pilot symbols to the processor155 for channel estimation.

In addition, the symbol demodulator 145 receives a frequency responseestimated value for downlink from the processor 155, data-demodulatesthe received data symbols to acquires data symbol estimated values(which is estimated values of the transmitted data symbols), andprovides the data symbol estimated values to the Rx data processor 150.The Rx data processor 150 demodulates (i.e., symbol-demaps),deinterleaves, and decodes the data symbol estimated values to recoverthe transmitted traffic data.

Processing operations by the symbol demodulator 145 and the Rx dataprocessor 150 are complementary to processing operations of the symbolmodulator 120 and the Tx data processor 115 in the BS 105, respectively.

In uplink, the Tx data processor 165 of the UE 110 processes trafficdata to provide data symbols. The symbol demodulator 170 may receive andmodulate the data symbols and provide a stream of the symbols to thetransmitter 175. The transmitter 175 receives and processes the streamof symbols to generate an uplink signal. In addition, the Rx antenna 135transmits the generate uplink signal to the BS 105.

In the BS 105, an uplink signal from the UE 110 is received by the Rxantenna 130, and the receiver 190 processes the received uplink signalto acquire samples. Then the symbol demodulator 195 processes thesamples to provide pilot symbols and data symbol estimated values whichare received for downlink. The reception (Rx) data processor 197processes the data symbol estimated values to recover the traffic datatransmitted from the UE 110.

The processors 155 and 180 of the UE 110 and the BS 105 order (e.g.,controls, manipulates, manages, etc.) operations of the UE 110 and theBS 105, respectively. The processors 155 and 180 may be respectivelyconnected to the memories 160 and 185 which store program codes anddata. The memories 160 and 185 are respectively connected to theprocessors 155 and 180 and store an operating system, application, andgeneral files.

The processors 155 and 180 may be referred to as a controller, amicrocontroller, a microprocessor, a microcomputer, or the like. Theprocessors 155 and 180 may each be embodied by hardware, firmware,software, or a combination thereof. When an embodiment of the presentinvention is embodied by hardware, the processors 155 and 180 mayinclude application specific integrated circuits (ASICs), digital signalprocessors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), or the like which is configured to execute the presentinvention.

When an embodiment of the present invention is embodied by firmware orsoftware, firmware or software may be configured in the form of amodule, a procedure, a function, etc. which perform function oroperations according to the present invention. Firmware or softwareconfigured to implement the present invention may be included in theprocessors 155 and 180 or stored in the memories 160 and 185 and drivenby the processors 155 and 180.

Layers of a wireless interface protocol between wireless communicationsystems (network) of the UE 110 and the BS 105 may be classified into afirst layer L1, a second layer L2, and a third layer L3 based on threelower layers of an open system interconnection (OSI) model that is wellknown in a communication system. A physical layer belongs to the firstlayer L1 and provides an information transfer service through a physicalchannel. A radio resource control (RRC) layer belongs to the third layer(L3) and provides control radio resources between the UE 110 and anetwork. The UE 110 and the BS 105 may exchange RRC messages through awireless communication network and an RRC layer.

Throughout this specification, the processor 155 of the UE 110 and theprocessor 180 of the BS 105 perform an operation for processing signalsand data except for a function of receiving or transmitting signals bythe UE 110 and the BS 105 or a storing function. However, hereinafter,for convenience of description, the processors 155 and 180 will not bespecially stated. Unless the processors 155 and 180 are not stated, aseries of operations such as data processing but not the function oftransmitting or receiving signals and the storing function may beperformed.

FIG. 3 is a diagram illustrating concept of self interference.

As illustrated in FIG. 3, a signal transmitted from a UE is received byan Rx antenna of the UE and acts as interference. This interference hasdifferent characteristic from other interferences. According to thefirst characteristic, a signal that acts as interference may beconsidered as a completely known signal. According to the secondcharacteristic, power of a signal that acts as interference is very highcompared with a desired signal. Due to this point, even if a signal thatacts as interference is completely known, the interference cannot becompletely cancelled at a receiver. The receiver uses an analog todigital converter (ADC) in order to convert a signal received by thereceiver into a digital signal. In general, the ADC measures power of areceived signal, adjusts a power level of the received signal accordingto the measured power, quantizes the received signal, and then, convertsthe signal into a digital signal. However, since an interference signalis received with higher power than a desired signal, the signalcharacteristic of the desired signal is covered by a quantization levelduring the quantization, and thus, the signal cannot be recovered.

FIG. 4 is a diagram illustrating signal distortion due to quantizationerrors. FIG. 5 is a diagram illustration signal recovery whenquantization errors are low.

In FIG. 4, for example, quantization is assumed to be 4 bits. As seenfrom FIG. 4, when an interference signal has much higher power than adesired signal, if quantization is performed, even if the interferencesignal is cancelled, the desired signal is highly distorted. On theother hand, FIG. 5 shows an example in which an interference signal haslower power than a desired signal and the desired signal is recoveredafter the interference signal is cancelled. In this situation, a schemefor cancelling self-interference may be classified into 4 schemesaccording to a position in which the scheme is performed.

FIG. 6 is a diagram for explanation of a scheme for cancellingself-interference.

Referring to FIG. 6, the scheme for cancelling self-interference may beclassified into 4 schemes of a baseband IC scheme, an ADC IC scheme, ananalog IC scheme, and an antenna IC scheme according to a position inwhich the scheme is performed.

FIG. 7 is a diagram for explanation of an antenna IC scheme using adistance between antennas.

The antenna IC scheme can be implemented via a simplest method among allIC schemes and can be performed as shown in FIG. 7. That is, one UEcancels interference using three antennas and uses two antennas as a Txantenna and one antenna as an Rx antenna among the three antennas. Thetwo Tx antennas are installed at a distance difference corresponding toabout wavelength/2 based on the Rx antenna in order to receive a signaltransmitted from each Tx antenna as a signal, a phase of which isinversed, in terms of the Rx antenna. Accordingly, an interferencesignal among signals that are lastly received by the Rx antennaconverges toward 0. Alternatively, in order to inverse a phase of asecond Tx antenna, an interference signal can be cancelled using a phaseshifter as illustrated in FIG. 8 without using a distance betweenantennas as illustrated in FIG. 7.

FIG. 8 is a diagram for explanation of an antenna IC scheme using aphase shifter.

In FIG. 8, a left diagram illustrates a scheme for cancellingself-interference using two Rx antennas and a right diagram illustratesa scheme for cancelling interference using two Tx antennas. Theseantenna interference cancelling schemes are affected by a bandwidth andcenter frequency of a transmitted signal. As a bandwidth of atransmitted signal is reduced and a center frequency of the transmittedsignal is increased, interference cancelling performance is morestrengthened. FIG. 9 illustrates interference cancelling performanceaccording to a bandwidth and center frequency of a signal. Asillustrated in FIG. 9, as a bandwidth of a transmitted signal is reducedand a center frequency of the transmitted signal is increased,interference cancelling performance is more strengthened.

An ADC IC scheme will now be described. The ADC IC scheme refers to atechnology for easily cancelling interference by maximizing theperformance of an ADC that has a most serious problem in thatinterference cannot be cancelled even if an interference signal ispre-known. Although it is disadvantageous in that the ADC IC schemecannot be applied due to quantization bit limitation of the ADC foractual embodiment, self-interference cancellation performance requiredby a trend of gradually improving ADC performance may be lowered.

An analog IC scheme will now be described. The analog IC scheme is ascheme for cancelling interference prior to an ADC and cancelsself-interference using an analog signal. The analog IC scheme may beperformed in a radio frequency (RF) region or performed in an IF region.Interference is cancelled simply by phase and time-lagging a transmittedanalog signal and subtracting the analog signal from a signal receivedby an Rx antenna. The analog IC scheme is advantageous in that only oneTx antenna and one Rx antenna are required unlike the antenna IC scheme.However, since processing is performed using an analog signal,distortion may further occur due to complex implementation and circuitcharacteristic, thereby highly changing interference cancellationperformance.

A digital IC scheme will now be described. The digital IC scheme refersto a scheme for cancelling interference after an ADC and includes anyinterference cancelation performed in a base band region. As a simplestscheme is embodied by subtracting a transmitted digital signal from areceived digital signal. Alternatively, a UE or BS that transmitssignals using multi antennas may perform beamforming or precoding so asnot to receive the transmitted signal by an Rx antenna. In this regard,when these schemes are performed in a base band, these schemes may alsobe classified as digital IC. However, the digital IC is possible when asignal modulated in a digital form is quantized so as to recoverinformation about a desired signal. Accordingly, the digital IC isdisadvantageous in that an amplitude difference of signal power betweena desired signal and an interference signal obtained by cancellinginterference via one or more scheme among the above schemes needs to bewithin an ADC range in order to perform the digital IC.

FIG. 10 is a diagram illustrating a system obtained by combininginterference cancellation (IC) schemes.

The system illustrated in FIG. 10 is a system to which the above schemesare simultaneously applied and overall interference cancellationperformance is improved by combining interference cancellation schemesof respective regions. A scheme proposed according to the presentinvention proposes a series of procedures and frame structure forcancelling self-interference via a simplest antenna IC scheme among theabove schemes and improving overall cell throughput. However, when allof the analog, ADC, and digital IC schemes as well as the antenna ICschemes are applied, even if the scheme proposed according to thepresent invention, cell throughput may also be improved.

General analog cancellation is achieved via a subtraction method priorto a low noise amplifier (LNA) of a receiver using a signal after apower amplifier (PA) of a transmitter. This is because influence of asignal received by an actual antenna can be effectively reflected onlywhen the signal is extracted from a last node of the transmitter.

FIG. 11 is a block diagram for interference cancellation in anenvironment in which orthogonal frequency division multiplexing (OFDM)is used.

In the block diagram of FIG. 11, functional blocks according to theirpurposes may be added or omitted. In addition, a digital cancellationblock may be located after an IFFT unit. However, digital cancellationmay be performed directly using a digital signal before a DAC and afteran ADC or performed using a signal before an FFT and after an IFFT. Inaddition, FIG. 11 is a conceptual diagram for separating a Tx antennaand an Rx antenna and cancelling a self-interference signal, but anantenna configuration may be changed when FDR is possible using oneantenna.

A conventional analog cancellation scheme uses an attenuator and a delaydevice in order to reflect channel characteristic (time delay andsize/phase change until an actual RF signal is output from a Tx antennaand is received by an Rx antenna) between a Tx antenna and an Rxantenna. However, when an analog signal is manipulated using only theattenuator and the delay device, only linear influence can be reflected.This is because it is assumed that antenna characteristic and channelcharacteristic are linear. Frequency characteristic of an antenna isdefined as an S parameter.

FIG. 12 is a diagram illustrating S parameter characteristic of anantenna for a frequency.

In FIG. 12, a signal is radiated in a region with S11, the size of whichis small. That is, when an antenna having wideband characteristic isdesigned, the antenna has small S11 in a wide region as illustrated inFIG. 12, and when an antenna having narrow characteristic is designed,the antenna has very sharp S parameter characteristic.

In reality, when an antenna is physically designed, frequency versusantenna gain characteristic may be achieved as illustrated in FIG. 13.FIG. 13 is a diagram illustrating frequency versus antenna gaincharacteristic.

As illustrated in FIG. 13, a narrowband wireless communication systemtransmits a signal in a narrow frequency region and thus, it may bedetermined that frequency versus antenna gain is linear. However, when asignal is transmitted in a wide band, antenna gain forms a curve shapeand each frequency has nonlinear gain, and thus, nonlinearcharacteristic may be considered. In addition, the nonlinearcharacteristic of antenna gain may deepen according to an antennaconfiguration method or when the antenna has directivity. Thus, there isa need for an analog cancellation scheme that considers the nonlinearcharacteristic of a radio channel and antenna after a power amplifier(PA).

FIG. 14 is a diagram illustrating an example of an analog cancellationscheme.

In general, analog cancellation may be described with reference to FIG.14. That is, an analog cancellation unit divides a signal output from atransmitter, equalizing the signal and a self-interference signal usinga variable delay unit and variable attenuator for reflecting influenceof attenuation and time taken to receive a signal transmitted from a Txantenna by an Rx antenna, and then subtracts the self-interferencesignal at a receiver prior to an LNA. In this case, a system may bedesigned without using a band pass filter of a transmitter and areceiver due to system purpose. In addition, any scheme for generatingthe same signal as a self-interference signal using a plurality of fixeddelay units and a variable attenuator may also be included and appliedto the scheme proposed according to the present invention.

However, since the delay device and the attenuator are linear devices,the delay device and the attenuator cannot reflect nonlinearcharacteristic. Accordingly, the present invention proposes an analogcancellation scheme for forming the same signal as a self-interferencesignal and subtracting the self-interference signal to have excellentperformance compared with a conventional scheme by an analogcancellation unit in consideration of the nonlinear characteristic of anantenna and a radio channel.

When an input signal is distorted due to a nonlinear device, an outputsignal may be represented in the form of Taylor series or power seriesaccording to Expression 1 below.

$\begin{matrix}{{y(t)} = {\sum\limits_{k = 1}^{\infty}\;{b_{k}{x(t)}{{x(t)}}^{({k - 1})}}}} & \left\lbrack {{Expression}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In this case, x(t) is an input signal, y(t) is an output signal, b_(k)is a random constant, k is a degree, and t is time.

Alternatively, the output signal may be represented in the form ofVolterra series. In this case, the output signal may be represented inthe sum of the set of Expression 1 for random time q. Here, influence ofmemory effect can also be considered. Through this, nonlinear gain to afrequency can be effectively modeled and can be represented according toExpression 2 below.

$\begin{matrix}{{y(t)} = {\sum\limits_{q = 0}^{\infty}\;{\sum\limits_{k = 1}^{\infty}\;{b_{q,k}{x\left( {t - q} \right)}{{x\left( {t - q} \right)}}^{({k - 1})}}}}} & \left\lbrack {{Expression}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Here, x(t) is a signal that passes through a band pass filter at atransmitter, that is, an input signal before being transmitted throughan antenna at the transmitter. Assuming that the aforementionednonlinear characteristic of the antenna and radio channel is lower thanlinear characteristic, b_(k) is reduced as k is increased. Accordingly,a degree up to infinity does not have to be considered. In addition, ingeneral, the degree may be considered up to third to seventh degrees. Inaddition, when symmetric characteristic of an input signal is ensured,Expression 3 below may be obtained in consideration of only an oddnumber among degrees for generating an output signal (although theproposed scheme is not limited to the case in which only an odd numberis considered as a degree, only an odd number is considered forconvenience of description.).

$\begin{matrix}{{y(t)} = {\sum\limits_{k = 1}^{K}\;{b_{k}{x(t)}{{x(t)}}^{2{({k - 1})}}}}} & \left\lbrack {{Expression}\mspace{14mu} 3} \right\rbrack\end{matrix}$where K is a random natural number.

Alternatively, when the output signal is represented in the form ofVolterra series, Expression 4 below can be achieved. In this case, Qrefers to a random natural number including 0.

$\begin{matrix}{{y(t)} = {\sum\limits_{q = 0}^{Q}\;{\sum\limits_{k = 1}^{K}\;{b_{q,k}{x\left( {t - q} \right)}{{x\left( {t - q} \right)}}^{2{({k - 1})}}}}}} & \left\lbrack {{Expression}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Accordingly, blocks of FIG. 15 may be configured to generate andsubtract the same signal as a self-interference signal that is distorteddue to the nonlinear characteristic of an antenna and radio channel.

FIG. 15 is a block diagram for reflection of nonlinear distortioncharacteristic.

FIG. 15 illustrates components of an analog cancellation unit as blocks.In FIG. 14, the analog cancellation unit is formed between a node beforea signal passes through a band pass filter via a power amplifier (PA)and then is output through an antenna at a transmitter and a node priorto the band pass filter before the signal passes an Rx antenna at areceiver. In FIG. 15, only a degree corresponding to an odd number isconsidered in order to reflect nonlinear distortion and only third andfifth degrees are considered. In FIG. 15, an operation part |.|²represents squaring of an input signal x(n) and an operation part |.|⁴represents 4 squaring of an input signal x(n).

In addition, when a degree of a seventh degree or more or an even numberis considered, blocks may be configured by adding the degrees of FIG.15. When these blocks are applied to an analog cancellation scheme forcancelling self-interference, the corresponding cases may be representedaccording to the following diagrams. FIGS. 16 and 17 illustrate the casein which up to the tertiary nonlinear characteristics of an antenna anda radio channel are considered.

FIG. 16 is a diagram illustrating an analog cancellation scheme thatconsiders nonlinear characteristic of a self-interference signalaccording to the present invention. FIG. 17 is a block diagramillustrating an example of an analog cancellation scheme and illustratesthe case in which the analog cancellation scheme is extended, Q=2, andK=2.

Referring to FIG. 17, although FIG. 17 illustrates a variable delaydevice, a fixed delay device can be used for reduction of complexity.When the fixed delay device is used, the amount of fixed delay iscalculated so as to include an average value of a delay value ofexperimentally measured average self-interference. For example, when twofixed delays are used, an average delay value of self-interference is τ,a delay value of fixed delay 1 may be set to be slightly smaller than τand a delay value of fixed delay 2 may be set to be lightly greater thanτ.

Referring to FIG. 16, in order to calculate b₁ to b_(k), a preamble orpilot signal that is known between a transmitter and a receiver may beused. However, since a self-interference signal is generated in onedevice, any signal output to a Tx antenna, such as a data signal as thepreamble or pilot signal may be used as the preamble or pilot signal.When φ_(k) (x)=|x|^(2(k-1)) x is defined, Expression 5 below may beobtained.

$\begin{matrix}{{y(t)} = {\sum\limits_{k = 1}^{K}\;{b_{k}{\phi_{k}\left( {x(t)} \right)}}}} & \left\lbrack {{Expression}\mspace{14mu} 5} \right\rbrack\end{matrix}$

When Expression 5 above is represented in the form of matrix, Expression6 below may be obtained.φ_(k)(x)=[φ_(k)(x(t ₁)φ_(k)(x(t ₂)) . . . φ_(k)(x(t _(N)))]^(T)φ(x)==[φ₁(x)φ₂(x) . . . φ_(K)(x)]y=φb  [Expression 6]

Accordingly, in order to calculate a value ‘b’, pseudo-inverse may beused.b=(φ^(H)φ)⁻¹φ^(H) y

When the pseudo-inverse is calculated, the complexity of hardware may beseriously increased. Accordingly, an optimum coefficient b_(k) may becalculated via an iteration method. That is, b_(k) may be changedstep-wise for a predetermined training period and an optimum value maybe obtained using a difference between a transmitted signal and areceived signal. This structure may be represented as illustrated inFIG. 18.

Although not illustrated in FIG. 16, an analog cancellation unit mayfurther include a phase shifter that performs phase shift on a signaloutput from a variable delay unit. Then signals that pass through thephase shifter pass through the operation part |.|² and a multiplier (x).When the phase shifter is added, the characteristic of nonlinear phasechange of an antenna and radio channel may be well cancelled.

FIG. 18 is a diagram illustrating an analog cancellation scheme thatconsiders nonlinear characteristic of a self-interference signalaccording to the present invention.

As illustrated in FIG. 18, an analog cancellation unit may furtherinclude a control block for calculating a variable delay value and avalue b_(k). In FIG. 18, the control block may calculate an optimumdelay value and coefficient by comparing a transmitted signal and areceived signal using a pseudo-inverse scheme or an iteration scheme asa calculation method. Alternatively, a look up table (LUT) may be usedfor convenience of implementation.

FIG. 19 is a diagram illustrating an analog cancellation scheme thatconsiders nonlinear characteristic of a self-interference signalaccording to the present invention.

FIG. 19 illustrates an example using a look up table (LUT). Asillustrated in FIG. 19, analog cancellation may be performed inconsideration of nonlinear characteristic by mapping an optimumcoefficient corresponding to a degree to a size corresponding to squareof an input signal according to a table and outputting a response valueof the optimum coefficient.

In FIG. 19, a high degree method may also be embodied via parallel useof a look up table (LUT) in a method or scheme that considers degrees upto a third degree only, and the control block used in FIG. 19 may beadditionally used in order to adjust values of the LUT.

In FIG. 19, an analog cancellation unit is configured by assuming that asignal output from a Tx antenna is received by an Rx antenna along asingle path. However, since a signal output to the Tx antenna is alsoreceived by the Rx antenna along a multi-path, influence due to themulti-path needs to be reflected. When a signal is received along amulti-path, the analog cancellation unit configured in FIG. 19 may beadded in parallel between a transmitter and a receiver at each path. Theanalog cancellation unit may be added according to the multi-path, whichmay be applied to the block diagrams illustrated in FIGS. 14 and 17 aswell as in FIG. 19.

An object of the aforementioned scheme proposed according to the presentinvention is to completely recover a self-interference signal inconsideration of nonlinear characteristic of an antenna and radiochannel and to improve the usefulness of frequency dependent rejection(FDR) by subtracting a copied signal at a receiver to cancel aself-interference signal.

According to an embodiment of the present invention, a self-interferencesignal of nonlinear characteristic as well as a self-interference signalof linear characteristic between antennas may be completely cancelled inconsideration of nonlinear characteristic of an antenna and radiochannel to improve the usefulness of frequency dependent rejection(FDR).

The embodiments of the present invention described above arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obvious tothose skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim by asubsequent amendment after the application is filed.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the inventions. Thus, itis intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. An apparatus for cancelling a self-interferencesignal between a transmission antenna and a reception antenna, theapparatus comprising: a first self-interference signal cancellation unitfor cancelling a self-interference signal in consideration of a linearchannel between the transmission antenna and the reception antenna; asecond self-interference signal cancellation unit for cancelling aself-interference signal in consideration of nonlinear channelcharacteristic or linear characteristic of a radio channel between thetransmission antenna and the reception antenna; and a controller forcomparing a transmitted signal output from the transmission antenna anda received signal received by the reception antenna to provide a firstcoefficient to be applied to self-interference signal cancellation of alinear element in the first self-interference signal cancellation unitand a second coefficient to be applied to self-interference signalcancellation of a nonlinear element in the second self-interferencesignal cancellation unit.
 2. The apparatus according to claim 1, whereinthe second self-interference signal cancellation unit cancels aself-interference signal of a nonlinear element using the secondcoefficient.
 3. The apparatus according to claim 1, wherein the firstself-interference signal cancellation unit cancels a linearself-interference signal in consideration of time taken to receive thetransmitted signal by the reception antenna using the first coefficient.4. The apparatus according to claim 1, wherein the firstself-interference signal cancellation unit and the secondself-interference signal cancellation unit are configured in seriesbetween a receiver comprising the transmission antenna and a receivercomprising the reception antenna.
 5. The apparatus according to claim 1,wherein the controller is configured between the transmission antennaand the reception antenna.
 6. The apparatus according to claim 1,wherein the first self-interference signal cancellation unit, and thesecond self-interference signal cancellation unit are connected andconfigured in series between a node between a band pass filter and atransmission antenna at a transmitter and a node between a band passfilter and a reception antenna at a receiver.
 7. A method for cancellinga self-interference signal between a transmission antenna and areception antenna, the method comprising: comparing a transmitted signaloutput from the transmission antenna and a received signal received bythe reception antenna to provide a first coefficient to be applied toself-interference signal cancellation of a linear element and a secondcoefficient to be applied to self-interference signal cancellation of anonlinear element; cancelling a linear self-interference signal usingthe first coefficient; and cancelling a nonlinear self-interferencesignal using the second coefficient.
 8. The method according to claim 7,wherein the cancelling of the linear self-interference signal comprisescancelling a self-interference signal in consideration of time taken toreceive the transmitted signal by the reception antenna using the firstcoefficient.